Why Drop to Ground addon doesn't work?
Object.ray_cast changed in 2.77 to include a "hit" return value giving the success of the hit.

You need to replace lines 108 to 111 with

        hit, hit_location, hit_normal, hit_index = tmp_ground.ray_cast(lowest_world_co, lowest_world_co + down)
        if not hit:
            print(ob.name, 'didn\'t hit the ground')
            continue


Full edit here https://gist.github.com/batFINGER/820d385839a222e5a10c1b7f5cdbd0b7